The present invention relates to a transmittable light-scattering sheet useful for brightening a display screen of a liquid crystal display device, and a liquid crystal display device using the sheet.
The liquid crystal display device is utilized in the display segments of, for example, personal computers, word processors, liquid crystal televisions, chronometers, desktop calculators. Recent years have witnessed advances in the construction of infrastructures for telecommunications systems and in the network consolidation of information through the computer-communications equipment integration, such as internets. By network consolidation of information, the access to information is freed from restrictions as to time and place. It is advantageous for a utilization of such networks that small-size telecommunication equipments, for example, portable information terminals such as PDA (personal digital assistance) which have recently been developed and further-downsized mobile personal computers (PC) of reduced thickness and weight compared to notebook-sized personal computers (PC) are utilized. Since portability is required of the portable information terminal and the mobile PC, it is necessary to reconcile the need for a longer battery operating time with the need for reduced thickness and downsizing of communications devices. Therefore, display devices for use in the portable information terminal and the mobile PC must be of reduced thickness, reduced weight and low power consumption. For attaining these requirements, the display mode which is considered to be most promising is a reflecting mode liquid crystal display device. Particularly, in order to keep abreast with the increasing versatility of data accompanying the ever-continuing advances in multimedia, there is a demand for reflecting-mode liquid crystal display devices not only capable of color display and high image-quality (high-definition) display but also of low production cost.
As the reflecting mode liquid crystal display device, there are known various kinds of devices. For color display and high image-quality (high-definition) display, a device utilizing one polarizer plate is advantageously employed. For example, the R-OCB mode in which the liquid crystal layer is of the HAN (Hybrid Aligned Nematic) alignment has excellent characteristics such as low voltage, wider viewing angle, high-speed response, middle color rendition and high contrast. In such a reflecting type liquid crystal display device, since an incident light (natural light, ambient light) on a front surface is efficiently introduced into the liquid crystal layer and is reflected with a light reflecting electrode on back side of the liquid crystal layer, and the reflected light is diffused so that the visibility is not deteriorated, such a device is required to exploit a natural or ambient light sufficiently.
As the reflecting type Liquid crystal display device, in Japanese Patent Application Laid-Open No. 228887/1988 (JP-63-228887A) and Photofabrication Symposium ""92 sponsored by the Japanese Society of Printing, the fundamental technology about reflecting type liquid crystal display device, and the liquid crystal display device given an enlarged viewing angle of the display surface through the prevention of total reflection by means of adopting a surface-corrugated metal thin film as the lower electrode (back electrode) were introduced. Moreover, in lieu of such a manner with the diffusion reflector, there is also known a manner employing a transmittable light-scattering sheet (Japanese Patent Publication No. 8430/1981 (JP-61-8430B)).
In general, the proportion of loss of external light (reflected light) is large since a polarizer plate is employed in the reflecting type liquid crystal display. Particularly, in case of a color display device, the proportion of loss of external light (reflected light) is further increased since a color filter is used. Thus, a simple diffused light (e.g., a light diffused in a Gaussian distribution) cannot impart enough brightness to the display screen. In order to solve such problems, it is required that a diffused light is directed in a given direction (directed-type diffusion).
However, in case of directing a reflected light diffused by a diffusing-reflector, it is necessary to precisely control the shape and distribution of the surface irregularities of the reflector but this is a costly procedure.
Meanwhile, when the diffuse-reflected light is directed by utilizing a transmittable diffusing sheet, a method for imparting directionality to the diffuse-reflected light by means of a resin sheet as polymerized by utilizing holography is known (The synopsis of Lectures at Japanese Society of Liquid Crystal Science, 1998) but the production process is complicated and costly.
While, as a transmittable diffusing sheet obtainable at low production cost, there is known a transmittable diffusing sheet in a particle dispersion form having a droplet structure. However, in the sheet having the particle dispersion structure, the distribution of a diffuse light is close to the Gaussian distribution in principle. Thus, the diffused light can not be directed to an angle other than one in regular reflection and high brightness of the display image can not be realized.
The object of the present invention is, therefore, to provide a transmittable light-diffusing sheet capable of imparting directionality to a diffused light and a liquid crystal display device with the sheet.
It is a further object of the present invention to provide a transmittable light-diffusing sheet useful for enhancing the intensity of the diffused light over a broad angle range and a liquid crystal display device with the sheet.
The inventors of the present invention made intensive investigations to accomplish the above objects and found that the directionality can be imparted to a diffused light by forming a bicontinuous structure or an intermediate structure between the bicontinuous structure and a droplet structure inside a sheet which is obtained by phase separating a plurality of polymers owing to spinodal decomposition. The inventors of the present invention made further intensive investigations to accomplish the above objects and found that the directionality can be imparted to a diffused light with high intensity over a broad angle range, and the high brightness can be imparted to the whole display screen uniformly.
That is, a transmittable light-scattering sheet (a transmitting type light-scattering sheet) of the present invention comprises a plurality of layers, each having a bicontinuous structure composed of a plurality of polymers. The layers differ from each other in average period size of the bicontinuous structure. The sheet may comprise a first layer having a bicontinuous structure (e.g., the average period size of the bicontinuous structure of 1.5 to 4 xcexcm) and a second layer each having a bicontinuous structure (e.g., the larger average period size of the bicontinuous structure than that of the first layer by 0.5 to 8 xcexcm). The ratio of the period size of the first layer to that of the second layer may be the first layer/the second layer=1.3/1 to 4/1. In the sheet, the first layer may be next to the second layer. The ratio of the thickness of the first layer to that of the second layer may be the first layer/the second layer=1/1 to 10/1. The thickness of the sheet may be 3 to 300 xcexcm. The haze value of the sheet may be 20 to 80%. The bicontinuous structure may be formed by spinodal decomposition of a composition containing a plurality of polymers. The diffused light is directed by using such a sheet. For example, a reflector is disposed on the first layer side and a light is incident on the second layer side, a reflected and diffused light (sometimes referred to simply as a reflected light), which is reflected by the reflector and diffused through or via the sheet, may have a peak of the reflected light at a diffusion angle of 5 to 10xc2x0, or may have a broad area of the reflected light over a diffusion angle of 10 to 15xc2x0. A plurality of polymers constituting the bicontinuous structure may comprise a styrenic resin and an acrylic resin. The bicontinuous structure may have a phase separation structure of a plurality of polymers.
The present invention also includes a liquid crystal display device wherein the above sheet is disposed on light path of the liquid crystal cell, and a process for producing the transmittable light-scattering sheet, which comprises laminating a plurality of previously molded sheets varying in an average period size.
As used in this specification, the term xe2x80x9csheetxe2x80x9d means any two-dimensional product regardless of its thickness, thus inclusive of film.
The term xe2x80x9ca bicontinuous structurexe2x80x9d is used herein to include an intermediate structure between the bicontinuous structure and a droplet structure.